Acceptable drop in voltage

[sleepyluke]

I just installed my LED meter and know I am toward the end of my batteries. Running around yesterday I got to 30.3 is the lowest I saw on a hill. How low should it go before I worry or have to get batteries? 38.2 at rest. I like the idea of the meter but don't know what it is telling me thanks.

[cableguy_hd]

the meter will show a drop in voltage while under load...climbing a hill. Im 48v and mine will go down @ 43-45 when loaded. I would guess 30 is pretty low

[JohnnieB]

Hopefully you are speaking about a DVM type meter with seven-segment LED displays rather than a LED Bargraph type meter.
You mentions some specific voltages, so I suspect it is the former.

There are three types of battery voltages and a DVM type battery meter doesn't tell you which of the three it is measuring at the time. That's up to you to figure out, but once you know what the three are, it is pretty easy to understand what the meter is telling you.

1. At-Rest voltage. When a battery is charged, it acquires an artificially high voltage and the opposite is true when a battery is discharged. So to get an accurate SoC (State of Charge) estimate you have to let the batteries rest for about 12 hours after charging or about an hour after discharging.

For a rough Soc estimate while out and about to determine when to head back to wherever the charger is, a 5-10 minute rest after the cart stops, is usually long enough and the error in the estimate will be on the safe side.


2. On-Charge voltage. Voltage while a charging current is passing through the battery. Starts at whatever the voltage is when the charger is plugged into the cart and typically climbs into the 44V to 46V range when charger shuts off for a 36V pack.

3. Under-Load voltage. Battery voltage decreases while amps are being drawn and the more amps drawn, the greater the decrease.

When doing non-destructive testing, battery manufacturers typically use 1.75VPC (Volts per Cell) as the cutoff voltage, so that is a fairly safe goal to stay above while your cart is under heavy load. That works out to 31.5V for a 36V pack.

If the batteries are loaded down to 1.5VPC or below, irreversible plate damage can occur. That is 27V for a 36V pack.

The amount the battery's voltage drops per amps of current drawn varies with the AH capacity of the battery and SoC. Older batteries have a lower AH capacity due to the natural aging process and the SoC is lower on the return leg of the trip, so keep an eye on the meter while climbing hills on the way back to the house.

Also, Batteries have less capacity (AH) when the weather is cold. The loose about 10% of their AH capacity for every 15°F below 80°F.

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Taking your batteries down to 30.3V won't kill them, but it might knock a few days off their usable lifespan if done routinely.


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It takes some time to get used to, but the DVM battery meter is a very handy tool.

[sleepyluke]

Originally Posted by JohnnieB

Hopefully you are speaking about a DVM type meter with seven-segment LED displays rather than a LED Bargraph type meter.
You mentions some specific voltages, so I suspect it is the former.

Yes it is the specific voltage - bar type in a box on the shelf now
1. At-Rest voltage. When a battery is charged, it acquires an artificially high voltage and the opposite is true when a battery is discharged. So to get an accurate SoC (State of Charge) estimate you have to let the batteries rest for about 12 hours after charging or about an hour after discharging.

At rest is 38.2 volts

2. On-Charge voltage. Voltage while a charging current is passing through the battery. Starts at whatever the voltage is when the charger is plugged into the cart and typically climbs into the 44V to 46V range when charger shuts off for a 36V pack.

On charge was right around 45, then when with the charger not charging this morning, pack was at 38.0 and the charger had not reengaged.

3. Under-Load voltage. Battery voltage decreases while amps are being drawn and the more amps drawn, the greater the decrease.

When doing non-destructive testing, battery manufacturers typically use 1.75VPC (Volts per Cell) as the cutoff voltage, so that is a fairly safe goal to stay above while your cart is under heavy load. That works out to 31.5V for a 36V pack.

I got to about 30 - 30.5 when going up the road right by the house.
(4 year old batteries that are on their last leg)


Also, Batteries have less capacity (AH) when the weather is cold. The loose about 10% of their AH capacity for every 15°F below 80°F.

It was about 25 degrees

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Taking your batteries down to 30.3V won't kill them, but it might knock a few days off their usable lifespan if done routinely.

This hill is 100 yards from the house, and the cart can't go anywhere without clombing it first Think my wallet is about to get hurt with new batteries.


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It takes some time to get used to, but the DVM battery meter is a very handy tool.

I like the idea of it, and if i learn where my limits are, I think it is going to help the new pack greatly!

Thanks

[JohnnieB]

You say the cart can't go anywhere without climbing the hill. I take that to mean you are go downhill on the way back home. Sounds like you get to attack the hill with a fully charged battery pack.

I live on a hilltop and have a choice of a short steep climb or a longer, more gradual rise, to get back home. My batteries are fairly new (Nov-2012), but on the way home after a 10-15 mile trip, I can load them down to below 1.75VPC climbing the hill the house is on from either direction.

I accelerate before getting to the bottom of the hill and back off on the throttle to keep the voltage above 1.75VPC, letting the speed decrease to whatever it settles down to.

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Four year old batteries, 25° weather, a steep hill, 23" tires and a 500A controller. No wonder the battery voltage dropped so far.

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Give you batteries a boost charge right before taking the cart out and about, it'll handle the hill better.

Since replacing the batteries is in the foreseeable future, upping the pack voltage to 42V or 48V might be a good idea. Higher voltage means fewer amps drawn to climb the same hill, so the voltage won't drop as much.

[cableguy_hd]

And by looking at your signature you're lifted with back seat.... Go 48v when you get new batteries and you won't regret it. I did... The only real knock your wallet will take is the new charger. Get a DPI from scottyb. If it hasn't been suggested to you on here yet it will be at some point in time. you should be able to sell your old charger to help in the expense. The difference in 6v batteries to 8v may be 15$ or so. Its well worth if you can do it.


I just read johnnieb's last line after I typed all this.. lol Told you it would be suggested

[kojo]

You'll definitely notice the difference between 36 & 48v. Speed and torque both will be increased and possibly run-time, it's just according to what kind of batteries you HAD and what kind you're going to GET.

[sleepyluke]

Johnnie you are correct. The hill is roughly a 5-10 second climb and good for coming home.

And yes 48 is in my future. Just have to find the funds for it, last savings just went to an LED lightbar that I needed.